In recent years, saving energy resources has become an important concern in various fields, and the field of power supplies, for example, is no exception. More specifically, there has developed, for example, a need to further enhance the efficiency of switching power supplies.
A switching power supply whose output efficiency exceeds 90% has already been proposed in the art, but the current state of the art is approaching a limit when it comes to further enhancing the efficiency, because, for example, the power consumed by the switching transistor (switching device) used in the power supply becomes a bottleneck.
It is believed that the causes for the bottleneck due to the use of the switching transistor are the parasitic resistive component called the ON resistance of the transistor, especially the component residing on the current input side terminal of the transistor, and the capacitive component seen between each terminal of the transistor.
First, the problem attributable to the parasitic resistive component residing on the current input side terminal of the transistor occurs when the transistor is in the ON state. That is, when the transistor is turned on, allowing a current to flow through the transistor, the ON resistance of the transistor causes a voltage to develop between the current-carrying terminals of the transistor due to the ON resistance and the current in accordance with Ohm's law.
Here, since the power consumed by the transistor is equal to the product of the current flowing through the transistor and the voltage developed between the current-carrying terminals of the transistor, this power is not one that is recoverable as the output of the switching power supply, but is converted in the transistor into heat, resulting in a power loss.
Next, the problem attributable to the capacitive component seen between each terminal of the transistor occurs when the current and voltage abruptly change during the ON/OFF operation of the transistor. That is, during the ON/OFF operation of the transistor, the capacitance seen between each terminal of the transistor is charged and discharged.
Further, when the switching operation of the transistor starts, the charging/discharging of the capacitance causes a delay in the timing of switching operation between the voltage and current of the transistor. The larger the capacitance, the greater the timing delay.
As a result, the voltage is applied before the current becomes completely zero and, during this time, a power loss occurs, as in the case of the problem attributable to the parasitic resistive component residing on the current input side terminal of the transistor.
Generally, in the switching power supply, a field-effect transistor (FET) has been used as the switching device, and a typical example of such a transistor is a metal-oxide-semiconductor (MOS) transistor that uses a silicon material. The power loss described above has been a serious problem with this type of MOS (metal-oxide-semiconductor(-semiconductor)) transistor.
To reduce the power loss, a transistor that does not use silicon but uses a compound semiconductor has been developed for use in a switching power supply. Since many of the compound semiconductors have greater electron mobility and larger mutual conductance than silicon, the advantage is that not only is it possible to reduce the ON resistance, but the capacitance seen between each terminal of the transistor is also small.
However, the electrical characteristics in steady-state switching operation of a field-effect transistor that uses a compound semiconductor may vary depending on the ambient temperature or on the applied current and voltage; for example, the threshold voltage of the transistor may vary greatly.
More specifically, the threshold voltage of an n-channel transistor used in a switching power supply is normally expected to be positive, but in the case of a transistor using a compound semiconductor, the threshold voltage may shift into the negative side, depending on the operating conditions or operating environment.
The shift in the negative direction of the threshold voltage of such a field-effect transistor using a compound semiconductor occurs during the switching operation of the transistor; it is said that this phenomenon is strongly dependent on the charge/discharge of electrons from the electron trapping levels believed to exist at the semiconductor surface, the semiconductor-semiconductor interface, and the semiconductor-insulator interface, but at the present time, the details of the cause are not fully understood, nor is it possible to completely control the operation.
The variation of transistor threshold voltage occurs not only in compound semiconductor transistors, such as gallium-nitride high electron mobility transistors (GaN HEMTs), but more or less in various other transistors such as conventional MOS transistors.
The switching device compensation circuit according to any one of the embodiments described herein is widely applicable to various switching devices including compound semiconductor transistors such as GaN HEMTs and field-effect transistors such as MOSFETs.
Further, it will be appreciated that the switching device to be controlled is not limited to the transistor used as the switching device in the switching power supply, but may include switching devices used in various other electrical circuits.
In the related art, various types of switching power supply apparatus have been proposed that use field-effect switching transistors and that improve efficiency by reducing losses during light load periods.    Patent Document 1: International Publication Pamphlet No. WO 2005/078910